CLINICAL SCIENCE

aSenior LectuOulu UniversbResearch AOulu UniverscProfessor, DOulu Univers

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Outcome of zirconia single crowns made by predoctoral dentalstudents: A clinical retrospective study after 2 to 6 years of

clinical service

Ritva Näpänkangas, DDS, PhD,a Juha Pihlaja, DDS,b and Aune Raustia, DDS, PhDc

ABSTRACTStatement of problem. Zirconia has established its role as a reliable ceramic material for fixedprostheses.

Purpose. The purpose of this retrospective study was to evaluate the outcome of zirconia singlecrowns made by predoctoral students after 2 to 6 years of clinical service.

Material and methods. A cohort of 88 patients treated with zirconia single crowns (mean 3 crownsper patient, range 1 to 12 crowns) from 2007 to 2010 by predoctoral dental students was identified.The patients were invited to attend a clinical examination.

Results. Sixty-six participants (75%) took part in the clinical follow-up (30 women and 36 men;mean age 60.4 years, range 19 to 81 years). Altogether, 190 teeth with single crowns wereexamined, and the mean follow-up time was 3.88 years (1.85 to 6.04 years). The most commoncomplications were chipping of veneering porcelain (4%) and loss of cementation (4%). Thesuccess rate of the zirconia single crowns after 2 to 6 years was 80% and the survival rate 89%.

Conclusions. Zirconia crowns can be successfully used in predoctoral dental education. The successrate of zirconia single crowns after 2 to 6 years was 80% and the survival rate was 89%, inaccordance with previous studies. (J Prosthet Dent 2015;113:289-294)

Zirconia has been establishedas a reliable ceramic materialfor fixed prostheses in severalfollow-up studies.1-11 Thesurvival rate of zirconia hasbeen shown to be similar tothat of metal ceramics for bothcrowns12 and partial fixeddental prostheses (FDPs).13-18

Traditional metal ceramic res-torations are still the safestoption in patients withbruxism, because studies haveshown that the veneeringporcelain of ceramic restora-tions has a higher rate offracture than that of metal

ceramic restorations.19-21

Zirconia has better mechanical properties than otherceramics, including alumina, glass ceramics, and lithiumdisilicate.22 Zirconia is biocompatible and esthetic, andcan withstand both tensile and compressive stress.22,23

The strength of zirconia allows its use in anterior,premolar, and molar areas.6 Some studies have shown ahigher risk of complications for ceramic crowns inthe molar area than in anterior or premolar teeth,24 but inother studies no difference was seen.5-7 Compared with

rer, Department of Prosthetic Dentistry and Stomatognathic Physiology, Inity Hospital and University of Oulu, Finland.ssistant, Department of Prosthetic Dentistry and Stomatognathic Physiologity Hospital and University of Oulu, Finland.epartment of Prosthetic Dentistry and Stomatognathic Physiology, Instituteity Hospital and University of Oulu, Finland.

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lithium disilicate, the bond between the resin cement andthe zirconia may be weaker.25 A phosphate monomer isbeneficial in bonding.26 The retention and resistance ofzirconia crowns have to be ensured with adequate prep-aration of the abutment tooth. Another disadvantage ofzirconia is a high prevalence of chipping fractures in theveneering porcelain,20,21,27 even though the techniquesand materials of veneers have been improved.28-30

One option for restorations is to use monolithic

stitute of Dentistry, University of Oulu; and Medical Research Center, Oulu,

y, Institute of Dentistry, University of Oulu; and Medical Research Center, Oulu,

of Dentistry, University of Oulu; and Medical Research Center, Oulu,

289

Figure 1. Zirconia single crowns restoring maxillary incisors and canineteeth.

Clinical ImplicationsZirconia single crowns can be used with adequatesurvival and success in the predoctoral dentalstudent clinic.

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anatomic-contour zirconia without veneering porcelain.31

A framework fracture is a less frequent complication inzirconia restorations.32,33

The marginal gap of zirconia restorations has beenshown to be equal to that of metal ceramics.34,35 Themarginal gap has varied amongdifferent zirconiamaterialsbut has always been clinically acceptable.36-39 Anotherissue concerning zirconia restorations is the wear of bothzirconia and the antagonist enamel. However, zirconiawith adequate surface finish (polishing, glazing) resultedin the least wear of the antagonist enamel among thevarious dental materials; a smooth surface of zirconia canbe obtained with adequate polishing.40

In predoctoral dental education, the techniques andmaterials need to be reliable, because teaching the basicprinciples of preparation and procedures of prosthetictreatment should be the focus. In addition to the strengthof zirconia, the willingness to use metal-free, moreesthetic, and less expensive material than metal ceramicsalso encourages its use in predoctoral dental educationalongside metal ceramics. The hypothesis of the studywas that zirconia is also suitable for fixed prostheses inpredoctoral dental education. The aim of the study was toevaluate the outcome of zirconia single crowns in a 4-year clinical retrospective follow-up study.

MATERIAL AND METHODS

The study protocol was approved by the Ethical Com-mittee of the Northern Ostrobothnia Hospital District(100/2013). A total of 88 patients were treated with 268zirconia single crowns (mean 2.9 crowns per patient,range 1 to 12 crowns) between 2007 and 2010 by pre-doctoral dental students (Fig. 1). All patients were invitedto a clinical examination, and 66 patients (75%) took partin the study. Nine patients did not attend and did notcontact us, 11 patients had moved away (not invited),and 1 patient had died.

Before prosthodontic treatment, the patients receivedperiodontal treatment, caries control, and endodontictreatment, as well as occlusal adjustment if needed. Allthe preparations were made in accordance with inter-national treatment guidelines.41 Composite resin (Z250;3M ESPE) was used for foundation restorations whenneeded, and a fiber post (RelyX Fiber Post; 3M ESPE) wasused in endodontically treated teeth. All the treatmentprocedures were performed under the supervision ofprosthodontists.

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Before the definitive cementation of the zirconiasingle crowns, their esthetic appearance and occlusionwere evaluated. The clinical instructor evaluated the fit ofthe restoration, and the patients were asked whetherthey were satisfied with the color and esthetics. Therestorations were bonded with dual-polymerizing, self-adhesive universal resin cement (RelyX Unicem; 3MESPE), according to the manufacturer’s instructions.

The zirconia frameworks in the single crowns werefabricated from Zirkonzahn Zirconia (Zirkonzahn),NobelProcera Zirconia (Nobel Biocare), or Prettau Zir-conia (Zirkonzahn). With Zirkonzahn Zirconia and Pre-ttau Zirconia, the frameworks were fabricated by manualmilling and designed for uniform thickness of theveneering porcelain layer. The minimal thickness of theframework was 0.4 mm, and the veneering porcelain (GCInitial Zr; GC Europe) was layered on the frameworks.For NobelProcera Zirconia, the frameworks were fabri-cated with computer-aided design/computer-aidedmanufacture (CAD/CAM) and designed for uniformthickness of the veneering porcelain layer. The minimalthickness of the frameworks in the single crowns was 0.4mm in the anterior area and 0.7 mm in the posterior area.The veneering porcelain (VITA VM 9; VITA Zahnfabrik)was hand layered on the frameworks.

All the follow-up examinations were carried out bythe same prosthodontist (R.N.). The participants wereasked their opinions about esthetics, color match, con-tour, gloss, hypersensitivity to cold or heat, gingivalbleeding, pain related to the single crowns, and whetherthey noticed bruxism. At the clinical examination, theperiodontal condition (plaque accumulation/plaque indexand bleeding on probing/sulcus bleeding index accordingto Silness and Löe42and Mombelli et al43), the location ofthe crown margins related to the gingival margins, caries,or restorations in the crown margin, the presence ofendodontic treatment through the prosthetic crown, andthe status of the contralateral tooth were evaluated. Thedifference between the plaque accumulation and

Näpänkangas et al

Table 1. Indications for 204 single zirconia crowns

Indication n %

Large restoration 81 40

Poor esthetics 31 15

Unstable occlusion 26 13

Fractured restoration or tooth 22 11

Occlusal wear 13 6

Abutment tooth for PRDP 13 6

Old crown 12 6

Periodontal reasons 6 3

TOTAL 204 100

PRDP, partial removable dental prosthesis.

18 17

45

40

35

30

25

20

15

10

5

0

0

5

10

15

20

25

30

35

40

45

16 15 14 13 12 11 21 22 23 24 25 26 27 28

48 47 46 45 44 43 42 41 31 32 33 34 35 36 37 38

PonticsAbutments

PonticsAbutments

Figure 2. Distribution of single zirconia crowns in dentition.

April 2015 291

bleeding on probing in the abutment teeth and contra-lateral teeth were analyzed with the Fisher exact test(a=.05). Porcelain fractures were recorded and classifiedaccording to Anusavice.44 Any dental treatment per-formed at the Institute of Dentistry after the prosthetictreatment was determined from the patient records(Effica; Tieto).

The longevity of the single zirconia crowns wasmeasured from the day the crown was cemented to theday of a complication, or, if no complications occurred, tothe day of the clinical examination. A Kaplan-Meiersurvival analysis was performed on the basis of thesefacts and the survival percentage was counted at thepoint of 4 years because of the low number of singlecrowns after that time. A successful single crown wasdefined as a crown that had remained unchanged overthe observation period (according to Tan et al45). A sur-vived single crown was defined as a crown that was insitu at the examination visit, regardless of its rece-mentation or porcelain fractures (according to Tanet al45).

RESULTS

Altogether, 30 women and 36 men were examined. Meanage was 60.4 years (19 to 81 years). Mean follow-up was3.88 years (1.85 to 6.04 years). Altogether, 204 teeth wereprepared for crowns – Zirkonzahn 164 crowns (80%),Procera 21 crowns (21%), and Prettau 19 crowns (9%).The mean number of crowns was 2.9 crowns per patient(range 1 to 10 crowns).

The most common indication for a zirconia singlecrownwas a large existing restoration in the tooth (81/204,40%) (Table 1). Other common indications were pooresthetics (31/204, 15%) and unstable occlusion (26/204,13%). The distribution of zirconia single crowns is pre-sented in Figure 2. The opposing dentition was the pa-tient’s own teeth in 82% of restorations, a crown or partialFDP in 12%, a complete denture in 4%, and a partialremovable dental prosthesis in 2% of the restorations.

One participant had lost all 6 zirconia crowns becauseof loss of cementation (1 crown), because of root fracture

Näpänkangas et al

(2 crowns), and because of a change in the treatmentplan for the removable prosthesis (3 crowns). Oneparticipant lost 1 molar tooth with a crown because ofroot fracture, and thereafter 3 other adjacent teeth withcrowns were prepared for the abutment teeth of a partialfixed dental prosthesis. The number of individuals in theclinical examination remained at 64. In addition, 2 lostcrowns were attributed to cementation and 2 to peri-apical endodontic infections. The number of crowns inthe clinical follow-up remained 190.

Most of the participants were satisfied with the es-thetics (63/64 participants), color-match (61/64), contour(61/64), and gloss (64/64) of the zirconia single crowns.Three participants suffered from hypersensitivity to cold,and 3 of 64 participants noticed gingival bleeding. Oneparticipant experienced pain. Self-reported bruxism wasreported by 26 of 64 participants.

More plaque was seen in contralateral teeth than inteeth with zirconia crowns, but the difference was notstatistically significant (P=.376) (Table 2). Morebleeding on probing was seen in teeth with zirconiacrowns than in contralateral teeth (P=.012) (Table 3).

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Table 3. Bleeding on probing (sulcus bleeding index) around abutmentteeth (n=190) and contralateral teeth

Bleeding on ProbingAbutmentTeeth (%)

ContralateralTeeth (%)

No bleeding when a periodontal probe is passedalong the gingival margin

49 39

Isolated bleeding when a periodontal probe ispassed along the gingival margin

40 60

Confluent bleeding when a periodontal probe ispassed along the gingival margin

11 1

P=.012; Fisher exact test.

Figure 3. Porcelain fracture in zirconia single crown for maxillary leftcanine (Grade 1 according to Anusavice43). Fracture did not affectfunction or esthetics and surface has been polished. Figure 4. Porcelain fracture in zirconia single crown for maxillary right

incisor (Grade 1 according to Anusavice43). Fracture did not affectfunction or esthetics and surface has been polished. Bleeding on prob-ing was seen in abutment tooth; location of crown margin wassubgingival.

Table 2. Plaque accumulation (Plaque index) in abutment teeth (n=190)and in contralateral teeth

Plaque IndexAbutmentTeeth (%)

ContralateralTeeth (%)

No detection of plaque 77 41

Plaque only recognized by running a probeacross the marginal surface of the crown

17 56

Plaque can be seen by the naked eye 6 3

P=.376; Fisher exact test.

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The location of the crown margin was subgingival in43% of the single crown, marginal in 53%, andsupragingival in 4%.

Caries were not found and endodontic treatment wasnot performed through the prosthetic crown in any of theteeth with zirconia single crowns examined at follow-up.The most common complications were chipping of theveneering porcelain (4%) (Figs. 3, 4) and loss of cemen-tation (4%) (Table 4). According to Anusavice,44 3 por-celain fractures were grade 3 (severe chipping fracturesrequiring replacement of single crowns) and 6 porcelainfractures were grade 1 (fracture surfaces were polished;fractures did not affect function or esthetics). The successrate was 80%, and the survival rate was 89% after 4 years(Fig. 5).

DISCUSSION

The survival rate of zirconia single crowns made bypredoctoral dental students in the present study was89%, in accordance with previous clinical follow-upstudies, which found survival rates of 97.3% (5 years,ceramic restorations),5 98.1% (5 years),10 88.8% (5years),7 and 98.2% (3 years).9 The success rate (80%)here was lower than that reported earlier by Monacoet al10 (94.3% after 5 years). The limitation of the studywas that the zirconia crowns were performed bypredoctoral dental students and this may affect the sur-vival of the single crowns. Parafunctional habits andendodontically treated teeth pose a greater risk for fail-ures.5,10,20 Thus, complications may accumulate in the

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same individuals and, in the case of complication in 1crown, the risk for a complication in another crown mayincrease in an individual with multiple crowns. Earlyrepairable chipping of veneering porcelain may increasethe risk for unrepairable porcelain damage later.13,14 Theindications for choosing a material between ceramics,metal ceramics, or high-gold alloys must be well indi-cated in individual situations.

Several attempts have been made to improve thefracture rate of veneering porcelain in zirconia restora-tions. Anatomically designed frameworks have beenshown to result in a lower rate of porcelain fractures.28,31

Guess et al29 showed in an in vitro study that anatomi-cally shaped monolithic lithium disilicate crowns (IPS e-max CAD) resulted in fatigue-resistant crowns comparedwith hand-layer, veneered zirconia crowns (IPS e-maxZir-CAD/Ceram). The tooth should be adequatelyreduced to provide sufficient thickness of the veneeringporcelain. Occlusion must be evaluated properly, becausethe number and distribution of occlusal contact pointshave been shown to significantly influence the stressesinduced by occlusal forces in ceramic restorations.19 Inaddition, parafunctional habits have to be taken intoconsideration in treatment planning.20

It has been shown that biofilm formation on varioustypes of dental ceramics differ significantly; in particular,

Näpänkangas et al

~~

~~Cu

mul

ativ

e Su

rviv

al (%

)

100

80

60

0

0 2 4Time (y)

Figure 5. Survival rate in 204 zirconia single crowns was 89% after 4years.

Table 4. Complications in 204 zirconia single crowns

Complication

ZirkonzahnZirconia

NobelProceraZirconia

PrettauZirconia Total

n % n % n % n %

No complication 138 84 17 82 18 95 173 85

Porcelain fracture 7 4 2 9 0 0 9 4

Loss of cementation 5 3 2 9 1 5 8 4

Loss of crown

loss of cementation 3 2 0 0 0 0 3 2

root fracture 3 2 0 0 0 0 3 2

periapical endodontic infections 2 1 0 0 0 0 2 1

change in treatment plan 6 4 0 0 0 0 6 3

Total 164 100 21 100 19 100 204 100

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zirconia exhibited low plaque accumulation compared toglass-ceramic and a lithium disilicate glass-ceramic.46

Studies have also shown variations in plaque formationamong enamel and restorative materials, but it has beenconcluded that instructing the patient to maintain properoral hygiene and home care is more important than thechoice of restorative material.47

On the contrary, the difference between teeth withcrowns and contralateral teeth in terms of gingival bleedingwas not as clear as the amount of plaque would show.Plaque and gingivitis is obviously one reason for gingivalbleeding, but a subgingival crown margin can also causegingival bleeding, especially when the biological width hasbeen disturbed. Ideally the restoration margin should beplaced no more than 0.5 mm subgingivally to prevent theencroachment of biological width and the chronic peri-odontal inflammation.48 In addition, localized gingival irri-tation can be caused by excess cement, whichwas shown tobe a common early complication of single crowns.49

Some differences were seen in the number of compli-cations between the zirconia systems, but the number ofZirkonzahn Zirconia and Prettau Zirconia was quite low in

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this study, and no comparison between the systems wasdone. However, zirconia systems may exhibit differences,particularly in terms of mechanical failures, marginaladaptation, and color matching,36,39 although zirconiafabricated by CAD/CAM demonstrated a similar andacceptable marginal fit when compared with metal ce-ramics.34 It is important to record the manufacturers ofmaterials in patient files, because in the case of a compli-cation or a possible warranty issue, the material must beidentified.

CONCLUSIONS

Zirconia can be successfully used in predoctoral dentaleducation. The success rate of the zirconia single crownsafter 4 years was 80%, and the survival rate was 89%, inaccordance with previous studies.

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Corresponding author:Dr Ritva NäpänkangasUniversity of OuluBox 5281, FIN-90014 OuluFINLANDEmail: ritva.napankangas@oulu.fi

Copyright © 2015 by the Editorial Council for The Journal of Prosthetic Dentistry.

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